Doug Kerr
Well-known member
On several occasions, Bart has mentioned the Photoshop option "Blend at gamma x", in particular when it is set to "Blend at gamma 1.0".
I thought I would discuss this a little.
We will start by assuming that that option (found in Edit>Color Settings) is off. In our mode, we have a file with two layers. In the upper layer we have a uniform field with R=250. (We will only watch the "R" channel here; the same story would work for the other two.) In the lower layer we have a uniform field with R=100.
On the upper layer, we have a pixel mask, set to produce a transparency of 0.4.
What will be the R value of the composite image? This table will show how we can reckon that:
We see that for the upper layer, where R=250, the mask transparency, 0.4, by "diluting" the transparency of the pixels, gives them an effective R value of 0.4 x 250, or 100 as a contribution to the composite image.
The presence of these "0.4 potency" pixels in the image stack dilutes the visibility of the pixels on the lower layer; we only see them at "0.6 of par". Their actual R is 100, so they contribute to the R channel of the composite image to the extent of 60 units.
Thus the apparent R channel value of the composite image is 100+60=160 units.
But, because of the nonlinear nature of R, G, and B, the value R=160 does not properly represent the how the the composite image should look. We can see why better when we look at an arrangement that disposes of the problem.
Here, we look at the image buildup when we have turned on Blend at gamma 1.0:
Column Y shows the relative luminance (for the red channel component) represented by the two R values (this is based on the sRGB color space). (The scale of Y is 0-1.0.)
We really want to include 0.40 of the original luminance (red channel) of the upper image (because of the t=0.40 mask), and to see 0.60 of the original luminance of the lower image. These luminance values are shown in column Y'. Their sum would be the (red channel) luminance we would properly see in the composite image.
That luminance (0.458) would be represented by R=180. So the program (in this mode) makes the R value for the composite image 180. And all is well.
Why does Photoshop even include the other option? It points out that many other applications (poor ignorant things) work that way.
Bart suggests that, unless there is some reason to do otherwise, we keep Blend at gamma 1.0 turned on. Sounds good to me.
Best regards,
Doug
I thought I would discuss this a little.
We will start by assuming that that option (found in Edit>Color Settings) is off. In our mode, we have a file with two layers. In the upper layer we have a uniform field with R=250. (We will only watch the "R" channel here; the same story would work for the other two.) In the lower layer we have a uniform field with R=100.
On the upper layer, we have a pixel mask, set to produce a transparency of 0.4.
(Assuming that the active RGB color space and the nominated grayscale color space have the same "gamma", we could do that by painting the mask with RGB=102,102,102. But it doesn't matter here how we made that mask.)
What will be the R value of the composite image? This table will show how we can reckon that:
We see that for the upper layer, where R=250, the mask transparency, 0.4, by "diluting" the transparency of the pixels, gives them an effective R value of 0.4 x 250, or 100 as a contribution to the composite image.
The presence of these "0.4 potency" pixels in the image stack dilutes the visibility of the pixels on the lower layer; we only see them at "0.6 of par". Their actual R is 100, so they contribute to the R channel of the composite image to the extent of 60 units.
Thus the apparent R channel value of the composite image is 100+60=160 units.
But, because of the nonlinear nature of R, G, and B, the value R=160 does not properly represent the how the the composite image should look. We can see why better when we look at an arrangement that disposes of the problem.
Here, we look at the image buildup when we have turned on Blend at gamma 1.0:
Column Y shows the relative luminance (for the red channel component) represented by the two R values (this is based on the sRGB color space). (The scale of Y is 0-1.0.)
We really want to include 0.40 of the original luminance (red channel) of the upper image (because of the t=0.40 mask), and to see 0.60 of the original luminance of the lower image. These luminance values are shown in column Y'. Their sum would be the (red channel) luminance we would properly see in the composite image.
That luminance (0.458) would be represented by R=180. So the program (in this mode) makes the R value for the composite image 180. And all is well.
Why does Photoshop even include the other option? It points out that many other applications (poor ignorant things) work that way.
Bart suggests that, unless there is some reason to do otherwise, we keep Blend at gamma 1.0 turned on. Sounds good to me.
Best regards,
Doug